IO COLLEGE ZOOLOGY 



up of almost countless numbers. Non-living bodies possess no 

 unit of structure comparable to the cell. 



(5) Metabolism. — Animals are able to change food into 

 protoplasm; this process is termed metabolism (p. 19); growth 

 takes place by the addition of these particles of protoplasm 

 among the preexisting particles. This is growth by intussuscep- 

 tion. Non-living bodies are not metabolic, and, if they can be 

 said to grow at all, increase in size by the addition of particles 

 on the outside, that is, growth is by accretion. 



(6) Reproduction. — Animals are able to produce other ani- 

 mals like themselves. Non-living bodies cannot reproduce their 

 kind. 



(7) Irritability or Reactiveness. — Animals have the ability 

 of responding to changes in their environment. The change is 

 termed a stimulus, and the sum total of the animal's movements 

 is known as its behavior. Non-living orrects are not irritable. 



3. The Physical Basis of Life — Protoplasm 



Protoplasm is a term used by both zoologists and botanists to 

 designate the essential substance of which plants and animals 

 are composed. All living organisms are built up of protoplasm, 

 but no non-living object possesses any of this compound. Pro- 

 toplasm has been called by Huxley " the physical basis of life," 

 since all vital phenomena are due to its presence. 



There are several theories regarding its structure: A, the 

 alveolar theory, B, the reticular theory, and C, the granular 

 theory. According to the alveolar theory (Fig. 1), protoplasm 

 consists of two substances, one of which is in the shape of 

 spheres embedded in the other. The reticular theory (B) con- 

 siders protoplasm a network of living anastomosing fibers 

 among which are non-living substances such as water and fat. 

 The third theory (C) maintains that protoplasm is composed 

 of innumerable living granules variously arranged. It is still 

 uncertain which of these theories, if any, is correct. 



